The present invention relates to a main spindle device for a diesink type electric discharge machine which machines a hole shape into a workpiece by positioning an attached tool opposite a workpiece by a certain distance and applying a specific electric discharge voltage between the workpiece and the electrode. More particularly, the present invention relates to a main spindle device in which the main spindle may be controlled both by high speed rotation control and by high precision angle division control.
In general, main spindle devices for electric discharge machines are mounted on a machining head, the in and out motion of which is controlled in the plumb (vertical) direction by a servo motor on a column mounted on a bed, with the tool electrode advancing in the depth direction of the hole being machined in the workpiece. The tool electrode is attached at the main spindle device main spindle lower end, and the workpiece is placed on a surface plate on top of a table mounted on the head. A machining vessel placed around the surface plate is filled with a machining fluid which serves as an electric discharge machining medium, and machining of the workpiece is carried out therein in a submerged state.
It is advantageous when machining holes of various shapes using a main spindle device of this type to be able to move the tool electrode not only in and out in the advancing direction, but also to be able to turn the electrode or the workpiece in the high speed mode around the main spindle, or to angle divide in the rotational direction around the center axis of the main spindle. This makes it possible to perform electric discharge machining of machined holes having complex profiles by moving a round rod or other simple shaped tool electrode while rotating it with respect to the workpiece, or to attain a desired angle with respect to the workpiece using a tool electrode formed in a desired shape, such that machining can be performed at a desired angle of inclination. It is also possible, by combining main spindle angle division and perpendicular in and out motion, to machine complex machining holes such as screw holes.
Spindle devices in which such rotation control and angle division control are possible have been utilized for some time, and have been described in the patent literature. We shall now explain an example of a conventional main spindle device representative of such devices. In the explanation below, the motion control axis of the main spindle in the machining depth direction is referred to as the Z axis, the motion control axis in one axis direction on a plane perpendicular to the Z axis is referred to as the X axis, and the motion control axis in another axis direction perpendicular to the X axis on the aforementioned plane is referred to as the Y axis. Further, a rotation at approximately 1,000 rpm, and preferably at approximately 3,000 rpm is referred to as high speed rotation, and the axis rotational direction control is referred to as R axis control. Rotating the main spindle around the main spindle center to precisely position it at a desired angular position is referred to as angle division, and control of the rotational direction thereof in the control is referred to as C axis control. Rotation in that case is of course at a slow speed.
An example of a main spindle device in which the above-described operations are possible is depicted in FIG. 4. In FIG. 4 there is a main spindle device main unit 1; a main spindle 1A; a machining head 2; a servo motor 41; a rotary encoder 42; and a transfer mechanism 43 which transfers rotation of servo motor 41 to the main spindle. Explanation of the mechanism which moves the machining head 2 in and out, i.e. up and down, in the Z axis direction is omitted.
A rotary encoder 44 is attached to the servo motor 41 and the rotational speed of the servo motor 41 is detected. The rotary encoder 44 detects the rotation of a servo motor, and therefore a rotary encoder resolution of approximately 4,000 divisions (number of increments per degree) is sufficient. The rotary encoder 42 for angle division is attached to the main spindle 1A on machining head 2, and the angular position of the main spindle 1A is detected. The rotary encoder 42 is also variously referred to as the rotary scale and differs from devices normally placed on motors; a device having an extremely high resolution of, for example, 360,000 divisions is used. The reason such extremely high resolution rotary encoders or rotary scales are used is to respond to the particular workpiece requirements of high angle division precision. In particular, in electric discharge machining such as the screw hole machining described above, servo control may performed in which the tool electrode is controlled simultaneously in the Z axis and C axis directions while maintaining a fixed gap between the tool electrode and the workpiece, such that a much higher precision of angle division is required.
The transfer mechanism 43 comprises a coupling 43A affixed to the servo motor 41 output axis end, a pulley 43B affixed through the coupling 43A to the servo motor 41 axial end, a timing belt 43D running between the pulleys 43B and 43C, and a worm wheel 43F affixed to a machining head 2. Rotation of the servo motor 41 is transferred to the worm 43E, which causes the worm wheel 43F to rotate; the main spindle 1A is decelerated and rotates by means of the worm wheel 43F. As will be described below, that deceleration ratio is determined in accordance with the resolutions of the rotary encoders 42 and 44. Therefore in the case, as above, of a rotary encoder 42 having 360,000 divisions and a rotary encoder 44 having 4,000 divisions, a 1/90 device is selected.
When the main spindle 1A is rotated in the high speed mode in such a conventional device, a feedback signal from the rotary encoder 44 is used to control the servo motor 41 such that the main spindle rotates at a desired speed. In this situation, no feedback signal from the rotary encoder 42 is used.
Meanwhile, when angle dividing the main spindle 1A, the motor driver is switched in order to validate the feedback signal from the rotary encoder 42, which is used to control the angular position of the servo motor 41, while at the same time the feedback signal from the rotary encoder 44 is used to control the angular position of the servo motor. The reason for using two encoders in this manner is to permit a speed reducer to be interposed between the main spindle 1A and the servo motor 41. Due to the small amount of looseness, backlash, clutch slippage, etc. inherent in the speed reducer, control of the servo motor 41 does not immediately match that of the main spindle 1A, and therefore without feedback control is not stable from the respective rotary encoders for the servo motor 41 and the main spindle.
Feedback control using the two rotary encoders thus requires that the rotary encoder 44 resolution and the rotary encoder 42 resolution be matched. In this conventional example, a speed reduction ratio of 1/90 is selected, so the resolution of the 4,000 division rotary encoder 44 has a converted resolution of 4,000xc3x9790=360,000 at the main spindle 1A.
Another example of a main spindle device in which the above-described operations are possible is depicted in FIG. 5. Parts which are the same or similar as parts in the example described in FIG. 4 are given the same reference numerals. This example uses the same technical concept as the main spindle device disclosed in Laid Open Patent JP-H6-134624. In FIG. 5, there is depicted a main spindle device main unit 1; a main spindle 1A; a spindle 18 which is an integral piece with the main spindle 1A; a servo motor 51 for angle division; a rotary scale 52 attached around the spindle 18; and a brake device 54 which holds the main spindle 1A at an angle position after angle division has been performed. As in the previous embodiment, a high resolution of approximately 360,000 divisions is used for the rotary scale 53, and 4,000 divisions is used for the rotary encoder 52. A high rotation speed AC motor 55 is coupled to a speed reducer 56, which reduces speed at a specific speed reduction ratio based on the difference in number of teeth on inner and outer gears (not shown). The speed reduction ratio is 1/90, as in the previous embodiment. A clutch 57 separates the upper side of the speed reducer 56 from the spindle 1A. A jet flow unit 58 supplies a jet of machining fluid to the tool electrode.
When the main spindle 1A is rotating in the high speed mode, the servo motor 51 is separated from the spindle 1A by the clutch 57. At the same time the AC motor 55 is controlled such that the main spindle rotates at a desired speed. Excessively fast rotation and burning loss of the servo motor 51 are thus prevented. At the same time, when angle division on main spindle 1A is performed, the AC motor 55 is placed in an uncontrolled state, while the servo motor 51 is connected and controlled by the clutch 57. The motor driver for the servo motor 51 (not shown) is controlled; this motor driver controls the angular positioning of the servo motor 51 by means of the rotary scale 53 feedback signal, and controls the angular positioning of the servo motor 51 by means of the rotary encoder 52 feedback signal. Thus the main spindle 1A is positioned at a desired angular position. This example is similar to the previous conventional example for those points which are under feedback control by the two rotary encoders.
However, problems with connections arise in the structure of the above-described conventional examples. In the first conventional example, it is possible to switch the main spindle device between high speed rotation and angle division by means of a single motor. However, when the speed reduction ratio is increased in order to achieve a higher angle division precision, a problem arises in that the main spindle cannot turn in the high speed mode using a high speed reduction ratio. In other words, in this conventional example, the main spindle device requires that one or the other of the high speed rotation or high precision angle division functions be emphasized. In the second conventional example, the high speed rotation control and the angle division control can both be fully performed by switching between the two motors. However, the main spindle device requires 2 motors and 2 motor drives, as well as a speed reducer and other parts such as a clutch. The main spindle device is complex, having a large number of parts, in addition to being complex from a control standpoint.
Also, a speed reducer is used in both of the conventional examples, such that angle position control must be performed using a rotary encoder on the servo motor side, in addition to the requirement for a high resolution rotary encoder when performing high precision angular division.
An objective of the present invention, therefore, is to provide a main spindle device in which either high speed rotation control or high precision angle division control may be selected using a single motor and a single encoder without the need for a speed reducer and which is, as a result, of a simpler and more compact structure.
A main spindle device according to the present invention, which is one preferable embodiment for the purpose of achieving the and other objectives, may comprise the following elements; a machining head mounted so as to be able to travel in and out of a workpiece in a machining depth direction; a high output servo motor having a high rotor inertia in order to rotate the main spindle without decelerating; a high resolution angle position detector to detect the rotation speed and angular position of the servo motor for the main spindle; a numerical controller which outputs a switching signal to switch between high speed rotation of the above main spindle and angle division of the main spindle, while outputting a speed command signal in response to a desired rotational speed when turning the main spindle in the high speed mode, and outputting a desired angle position command signal when angle dividing the main spindle; and a motor driver which performs closed loop control of the high rotational speed of the main spindle using the speed command signal and a feedback signal from the angle position detector, while also performing closed loop control of the main spindle angle position using at least the angle position command signal and the feedback signal from the angle position detector when angle dividing the main spindle.
In the main spindle device of this embodiment, the main spindle high precision angle division and high speed rotation are controlled by a single motor, a single motor driver, and a single encoder. The result is a simple structure in which the main spindle device does not require a speed reducer.
In another preferred embodiment, a main spindle device machine according to the present invention comprises a main spindle placed on a machining head mounted so as to be movable in and out of a workpiece in a machining depth direction; a high rotor inertia, high output servo motor which rotates the main spindle without decelerating; a high resolution angle position detector mounted either on the servo motor or on the main spindle, which detects the rotational speed and angle position of the servo motor or the main spindle; a numerical controller which outputs a switching signal to switch between high speed rotation of the main spindle and angle division of the main spindle, while at the same time outputting a speed command signal in accordance with a desired rotational speed, and an angle position command signal for setting the desired angle when angle dividing the main spindle; and a motor driver which, when rotating the main spindle in the high speed mode, performs speed control through closed loop control of the servo motor by means of the speed command signal and a feedback signal, which is a signal from the angle position detector, the pulse count of which is reduced by a specified proportion. When angle dividing the main spindle, closed loop control of the main spindle angular position is performed by means of at least the angular position command signal and the feedback signal from the angular position detector.
In the main spindle device of this embodiment, high precision angle division and high speed rotation of the main spindle are controlled with a single motor, a single motor driver, and a single encoder, and a simple structure suffices wherein the main spindle device does not require a speed reducer. Also, the feedback signal pulse count is reduced, and the load on the motor driver is therefore reduced, as is the occurrence of errors.
In yet another preferred embodiment, the motor driver of the above-described main spindle device comprises a deviation output means which, when angle dividing the main spindle, feeds back a signal from the angle position detector to the angle position command signal and outputs the deviation thereof, and, when rotating the main spindle in the high speed mode, does not feed back the signal from the angle position detector to the rotation speed command signal; and a subtraction circuit which, when angle dividing of the main spindle, feeds back a signal from the angle position detector to the output of the position gain control element and outputs the deviation thereof, and, when rotating the main spindle in the high speed mode, feeds back the signal from the angle detector to the rotational speed command signal and outputs the deviation thereof; and a speed gain control element which, when angle dividing the main spindle or when rotating in the high speed mode, controls the outputs from the subtraction circuit at the respective desired speed gains and supplies an output.
According to this embodiment, when performing high speed control of the main spindle, a single servo motor controls the rotational speed by means of a feedback signal from a single angle position detector, while at the same time, when controlling the main spindle by high precision angle division, angular position is controlled by a single servo motor using the feedback signal from a single angle position detector.